Biochemistry of the Sliding Filament Model

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The Power Stroke

  • Myosin head attaches to actin filaments forming cross-bridge
  • Head group bends, thin filament pulled along to overlap more with thick thilament.
    -This is the Power Stroke!
    -ADP and Pi are released
  • Cross-bridge then broken as new ATP attaches to myosin head
  • Head group moves backwards, ATP is hydrolysed to ADP and Pi
    - Can form cross-bridge with thin filament further along and bend again 
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Calcium Ions and Muscle Contraction

  • Binding sites for the myosin head group are covered by tropomyosin subunits
  • Myosin cannot bind to these meaning cross-bridges can't be formed and cotnraction cannot occur
  • When a action potential arrives, calcium ions are released from the sarcoplasmic reticulum in sarcomeres.
  • Calcium ions diffuse through the sarcoplasm and bind to the troponin molecules
  • This changes the shape of the troponin
  • Tropomyosin moves away from the binding sites on the actin
  • Actin-myosin binding sites are uncovered
  • Cross-bridges can form
  • Results in powerstroke and muscle contraction
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Maintenance of ATP Supply

Three mechanisms by which ATP supplies are maintained:

  • Aerobic respiration in muscle cell mitochondria
  • Anaerobic respiration in muscle cell sarcoplasm
  • Transfer from creatine phosphate in the muscle cell sarcoplasm
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